Tantalum is a rare transition metal having a density of 16.6 g/cm3 and a melting point of 3017° C. Tantalum is hard, highly corrosion resistant, and is widely used as an alloying material, for example in making high strength alloyed steels or super alloys or carbide reinforced tools. The excellent corrosion resistance of tantalum means it is useful for making chemical process equipment and medical equipment and implants. The highest volume use of tantalum, however, is in the manufacture of capacitors. Tantalum capacitors tend to be small sized and of high capacitance, and have become vital components in cellular phones and personal computers.
The standard preparation route for tantalum powder is by reducing potassium tantalum fluoride (K2TaF7) with sodium and then water washing and acid leaching the product to remove the process salts. The powder is then dried and is known at this stage as primary or raw tantalum powder. Over the years there have been many variations on this basic process aimed at improving the process conditions and costs, and the powder's surface area. This route usually produces powders with average primary particle size ranging from 0.2 to 3.0 μm and surface areas of the order 1 to 3 m2/g. At this stage the oxygen content of the powder can be in excess of 3000 to 7000 ppm. In order further to reduce impurity content (by the removal of volatile elements) and improve the handling and sintering characteristics of the powder, the raw tantalum powder is subjected to vacuum heat treatment at temperatures up to 1200° C. This process thermally agglomerates the fine powder, and to reduce its particle size the resulting agglomerate must then be sieved. This produces a granulated material with an average particle size in the region of 10-100 μm. The steps of vacuum heat treatment and thermal agglomeration are necessary to reduce the impurity content and improve the handling and sintering characteristics of the Ta powder but have the significant disadvantage that the oxygen content is increased considerably; levels as high as 12000 ppm may be reached.
The aggregated tantalum powder must therefore undergo further processing in order to reduce the oxygen levels. Typically the powders are subjected to deoxidation with magnesium metal followed by acid leaching to remove the oxidation products (i.e. MgO) and further drying and classification stages. Powders produced by the standard potassium tantalum fluoride method (described above), which are thermally-aggregated and deoxidized, usually have a BET (Brunauer-Emmett-Teller) specific surface area of approximately 1 m2/g, mean primary particle size on the basis of specific surface area of around 400 nm, and a specific capacitance of approximately 50,000 CV/g. Such powders are produced in large amounts despite the complexity and cost of the fabrication procedure.
As the free energy change on reduction of tantalum oxide with active metals such as calcium, magnesium or aluminium is favourable, many attempts have been made to invent alternative process to the standard K2TaF7 process, which remove some of the well-known drawbacks mentioned above. The most successful to date is the reduction of tantalum oxide by reaction with gaseous magnesium. However, such methods are still disadvantageously complex multi-step processes wherein there is a first substantial reduction step followed by one or more deoxidation steps using similar reagents (gaseous magnesium) to achieve the desired oxygen levels.
Consequently, there is a long-felt need for a simpler process for producing tantalum, preferably a process capable of producing capacitor-grade (low oxygen content and high surface area) tantalum or Ta alloys.
Recently, it has proved possible to produce tantalum by direct reduction of a tantalum oxide feedstock. One such reduction process capable of producing tantalum is the Cambridge FFC electro-decomposition process (as described in WO 99/64638). In this method a solid tantalum oxide compound is arranged in contact with a cathode in an electrolytic cell comprising a fused salt. A potential is applied between the cathode and an anode of the cell such that tantalum oxide is reduced. In the FFC process the potential that reduces the tantalum oxide is lower than a deposition potential for a cation from the fused salt. For example, if the fused salt is calcium chloride then the cathode potential at which the tantalum oxide is reduced is lower than a deposition potential for depositing metallic calcium from the salt.
Other reduction processes for reducing feedstock in the form of cathodically connected tantalum oxides have been proposed, such as the polar process described in WO 03/076690.
A specific method of producing tantalum powders by electrochemical reduction is described in WO 2008/041007. This patent application describes a process for producing capacitor grade tantalum from a tantalum oxide feedstock.
Although tantalum has been successfully produced by direct electrolytic reduction of its oxide using the FFC Cambridge process, consistent and predictable control of the tantalum powder characteristics, such as morphology and particle size, has been difficult to achieve. It is an aim of the present invention to provide improved methods of producing tantalum, preferably tantalum powder having a consistent morphology and particle size.